Circadian rhythms are 24-hour cycles that are part of the body’s internal clock, running in the background to carry out essential functions and processes. One of the most important and well-known circadian rhythms is the sleep-wake cycle.
Different systems of the body follow circadian rhythms that are synchronized with a master clock in the brain. This master clock is directly influenced by environmental cues, especially light, which is why circadian rhythms are tied to the cycle of day and night.
When properly aligned, a circadian rhythm can promote consistent and restorative sleep. But when this circadian rhythm is thrown off, it can create significant sleeping problems, including insomnia. Research is also revealing that circadian rhythms play an integral role in diverse aspects of physical and mental health.
Circadian rhythms work by helping to make sure that the body’s processes are optimized at various points during a 24-hour period. The term circadian comes from the Latin phrase “circa diem,” which means “around a day.”
Circadian rhythms exist in all types of organisms. For example, they help flowers open and close at the right time and keep nocturnal animals from leaving their shelter during the daytime when they would be exposed to more predators.
In people, circadian rhythms coordinate mental and physical systems throughout the body. The digestive system produces proteins to match the typical timing of meals, and the endocrine system regulates hormones to suit normal energy expenditure.
The circadian rhythms throughout the body are connected to a master clock,sometimes referred to as the circadian pacemaker, located in the brain. Specifically, it is found in the suprachiasmatic nucleus (SCN), which is in a part of the brain called the hypothalamus. At different times of the day, clock genes in the SCN send signals to regulate activity throughout the body.
The SCN is highly sensitive to light, which serves as an critical external cue that influences the signals sent by the SCN to coordinate internal clocks in the body. For this reason, circadian rhythms are closely connected to day and night. While other cues, like exercise, social activity, and temperature, can affect the master clock, light is the most powerful influence on circadian rhythms.
Biological clocks help regulate the timing of bodily processes, including circadian rhythms. A circadian rhythm is an effect of a biological clock, but not all biological clocks are circadian. For instance, plants adjust to changing seasons using a biological clock with timing that is distinct from a 24-hour cycle.
When people talk about circadian rhythm, it’s most often in the context of sleep. The sleep-wake cycle is one of the most clear and critical examples of the importance of circadian rhythms.
During the day, light exposure causes the master clock to send signals that generate alertness and help keep us awake and active. As night falls, the master clock initiates the production of melatonin, a hormone that promotes sleep, and then keeps transmitting signals that help us stay asleep through the night.
In this way, our circadian rhythm aligns our sleep and wakefulness with day and night to create a stable cycle of restorative rest that enables increased daytime activity.
Research continues to uncover details about circadian rhythms, but evidence has connected them to metabolism and weight through the regulation of blood sugar and cholesterol. Circadian rhythms influence mental health as well, including the risk of psychiatric illnesses like depression and bipolar disorder as well as the potential for neurodegenerative diseases like dementia.
There are indications that circadian rhythms have an important influence on the immune system as well as processes of DNA repair that are involved in preventing cancer. Early-stage research indicates that circadian cycles can influence the effectiveness of anti-cancer drugs and that new medications may be able to harness biological clocks to kill cancer cells.
When circadian rhythm is thrown off, it means that the body’s systems don’t function optimally.
A distrubed sleep-wake circadian rhythm can give rise to serious sleeping problems. Without the proper signaling from the body’s internal clock, a person can struggle to fall asleep, wake up during the night, or be unable to sleep as long as they want into the morning. Their total sleep can be reduced, and a disrupted circadian rhythm can also mean shallower, fragmented, and lower-quality sleep.
In addition, studies have identified circadian rhythm disruptions as potential contributors to obstructive sleep apnea (OSA), a sleep disorder marked by repeated lapses in breathing. OSA reduces the body’s oxygen levels and causes numerous sleep interruptions through the night.
As a whole, a misaligned circadian rhythm can negatively affect sleep in many ways, increasing a person’s risk of insomnia and excessive daytime sleepiness. Given the essential role of sleep for productivity and overall health, there are often significant consequences when a person’s circadian rhythm is off.
Disruptions to circadian rhythm can occur over the short- or long-term. Experts have identified a number of types of circadian rhythm sleep-wake disorders (CRSWD) based on their characteristics and causes.
As this list demonstrates, there are diverse causes of circadian rhythm disorders. Some circadian disruptions are related to individual behavior, such as for travel or work, that puts sleep schedules out of whack with normal daylight exposure. Other disorders stem from an underlying issue that causes an inability to receive or process environmental cues that regulate the body’s master clock. In certain situations, genetic causes may be involved, or the cause may remain unknown.
While we don’t have full control over our circadian rhythm, there are healthy sleep tips that can be taken to try to better entrain our 24-hour sleep cycles.
These steps to improve sleep hygiene can be an important part of supporting a healthy circadian rhythm, but other steps may be necessary depending on the situation. If you have persistent or severe sleeping problems, daytime drowsiness, and/or a problematic sleep schedule, it’s important to talk with a doctor who can best diagnose the cause and offer the most appropriate treatment.